The present invention concerns a process for producing semiconductor devices, particularly a process for growing a tungsten film of a high-melting-point during a metallization process.
In metallization for semiconductor devices, a chemical vapor deposition (CVD) technique is attractive for making a uniform layer of good step coverage. Unlike the vacuum deposition, which suffers from a shadow effect and imperfect step coverage, the CVD techniqe can yield good step coverage even with great irregularity of the surface condition of the semiconductor devices, and also yields lower electrical resistivity.
One of the efforts of depositing metal for semiconductor devices by the CVD technique has been in deposition of a metal of high refractory characteristics such as tungsten, W. chemical symbol W. Tungsten also has a low resistivity (5.3 .mu..OMEGA.-cm), and may be grown from tungsten fluoride (WF.sub.6) by both pyrolytic and reduction reaction.
Referring to FIG. 1 for illustrating the result of a conventional process for selectively growing tungsten W, in a given region of a silicon layer 1 is formed a junction layer 2 by ion implantation, over which the junction layer 2 is covered by an insulating layer 3 which is selectively etched to form a pattern, through which tungsten 4 is grown by the CVD technique. In this case, in order to improve the adherence between the tungsten and the junction layer 2 in the initial growing step, plasma etching is performed on the surface of the layer by using NF.sub.3, SF.sub.6 gases, etc., thus roughening the surface, or silane (SiH.sub.4) reduction is performed after hydrogen (H.sub.2) reduction is performed so as to grow the tungsten thereon. However, the plasma etching results in a considerable corrosion in a contacting region 5 between the layer 2 and the tungsten, so that a large amount of the silicon in the contacting layer 2 is eroded causing the thickness of the contacting layer 2 to thin, thus increasing current leakage. Alternatively, if the tungsten is grown by SiH.sub.4 reduction, bad adherence occurs, which causes removal of the tungsten.